1. Field of the Invention
The present invention relates to the manufacture of parts such as complex geometry metals vanes and shrouds according to the technique known as lost wax casting.
2. Discussion of the Background
For the manufacture of vanes and shrouds for turbojet engines, such as rotor or stator parts, or structural parts according to this technique, a master pattern is prepared first of all, using wax or any other similar material easily disposable at a later stage. If necessary, several master patterns are gathered into a cluster. A ceramic mould is prepared around this master pattern by dipping in a first slip to form a first layer of material in contact with the surface thereof. The surface of said layer is reinforced by sanding, for easier bonding of the following layer, and the whole is dried: composing respectively the stuccowork and drying operations. The dipping operation is then repeated in slips of possibly different compositions, an operation always associated with the successive stuccowork and drying operations. A ceramic shell formed of a plurality of layers is then provided. The slips are composed of particles of ceramic materials, notably flour, such as alumina, mullite, zircon or other, with a colloïdal mineral binder and admixtures, if necessary, according to the rheology requested. These admixtures enable to control and to stabilise the characteristics of the different types of layers, while breaking free from the different physical-chemical characteristics of the raw materials forming the slips. They may be a wetting agent, a liquefier or a texturing agent relative, for the latter, to the thickness requested for the deposit.
The shell mould is then dewaxed, which is an operation thereby the material forming the original master pattern is disposed of. After disposing of the master pattern, a ceramic mould is obtained whereof the cavity reproduces all the details of the master pattern. The mould is then subjected to high temperature thermal treatment or “baked”, which confers the necessary mechanical properties thereto.
The shell mould is thus ready for the manufacture of the metal part by casting. After checking the shell mould for internal and external integrity, the following stage consists in casting a molten metal into the cavity of the mould, then in solidifying said metal therein. In the field of lost wax casting, several solidification techniques are currently distinguished, hence several casting techniques, according to the nature of the alloy and to the expected properties of the part resulting from the casting operation. It may be a columnar structure oriented solidification (DS), a mono-crystalline structure oriented solidification (SX) or an equiaxed solidification (EX) respectively. Both first families of parts relate to superalloys for parts subjected to high loads, thermal as well as mechanical in the turbojet engine, such as HP turbine vanes.
After casting the alloy, the shell is broken by a shaking-out operation, and the manufacture of the metal part is finished.
During the moulding stage, several types of shells may be used via several methods. Each shell should possess specific properties enabling the type of solidification desired. For example, for equiaxed solidification, several different methods may be implemented the one using an ethyl silicate binder, another using a colloïdal silica binder. For oriented solidification, the shells may be realised out of different batches, silica-alumina, silica-zircon or silica based batches.